Southern Pacific Communications Co. has dropped plans to build a $100 million corporate headquarters on a prime Bethesda site where about 6,000 employes ultimately might have worked for the long-distance-telephone firm.

The more had been heralded as a major corporate-recruiting coup by Montgomery County officials when it was announced in 1979 and as a significant change in focus for the subsidiary of Southern Pacific Co., the giant railroad holding company.

A company spokesman, Amy Fink, said the move had been "indefinitely postponed" because the company had decided that it needs the capital to expand its telephone network and to construct and operate a three-satellite network, recently approved by the Federal Communications Commission, that will cost at least $200 million.

Further, a Southern Pacific real estate subsidiary has sold the valuable property near Interstate 270 and Democracy Blvd. to Boston Properties Inc., turning it over after purchasing the site from Martin Marietta Corp.

The Boston-based real estate concern plans to build a 650,000-squarefoot "first class" office building on the site, a spokesman said. Officials of Boston Properties and Southern Pacific refused to discuss the terms of the sale.

Although Southern Pacific has expanded its operations in the Washington area, particularly in McLean and Rosslyn, industry sources say the decision not to move here also is based on company difficulties in the eastern market.

Other factors in the decision include an $11 million write-off for uncollectable billings during the 1980 third quarter and considerable resistence to the move from employes at the current SPC headquarters in Burlingame, Calif., industry sources said.

One industry analyst said the company has continued to lose money despite an agressive program of selling the cut-rate-telephone-service program to businesses and consumers.